1. Field of the Invention
The present invention relates to a method and apparatus thereof for encoding and decoding dither signals for reproduction of the halftone of a picture in the pseudo halftone, and more particularly to the improvement of the method and apparatus therefor capable of compressing the amount of data to be transmitted at the high compression efficiency.
2. Description of the Related Art
As is well known, a picture is indicated with the aggregation of pels in many types of picture indicating or reproducing apparatuses. In a facsimile device as a typical one of examples of such apparatuses, pseudo halftone indication using dithering technique is widely employed for reproduction of a halftone picture, because pels in the facsimile device can only indicate two levels of the tone, i.e., black and white.
In this technique, a series of multilevel picture signals x.sub.j of pels, which are obtained by scanning respective scanning lines, are binarized by using thresholds t.sub.j as follows. If x.sub.j .gtoreq.t.sub.j, a dither signal z.sub.j is made logical 1, whereas if x.sub.j &lt;t.sub.j, z.sub.j is made logical zero. The thus obtained two-level dither signals are reproduced on a recording paper as black or white pels, wherein one of the two levels is reproduced as a black pel and the other level as a white pel. If the thresholds t.sub.j are altered pel by pel in accordance with a certain rule, the ratio of black and white pels within a constant area of a reproduced picture varies so that the area can be indicated in the desired halftone.
The thresholds are arranged in a matrix, each element of which corresponds to one pel of a picture, and different brightness levels as the thresholds are distributed element by element Such a matrix is known as a dither matrix. FIGS. 1a to 1c show three examples of a 4.times.4 dither matrix with 16 threshold levels. Numerals indicated in elements of the matrices represent brightness levels set as the thresholds. Hitherto there are known various types of distributing the thresholds. Dither matrices of FIGS. 1a and 1b are known as a Bayer type and a screen type, respectively A matrix of FIG. 1c has no specific name. Since, however, the 16 level thresholds are arranged in a vortex line, it can be called a vortex type or a centralized type. A type of the dither matrices is selected in accordance with the nature of an original picture to be processed.
By the way, in an actual picture, brightness levels of neighboring pels will vary continuously, and hence those pels have the high correlation in the brightness level to each other. As shown in FIGS. 1a to 1c, however, since the thresholds are determined irrespective of the correlation in brightness between pels in an original picture, dither signals z.sub.j of the neighbouring pels, which are obtained as the result of processing by using any of the dither matrices as mentioned above, have the less correlation to each other. The less correlation between the dither signals of the neighbouring pels results in considerably deteriorating the data compression efficiency, when the dither signals are encoded by a Modified Huffman coding method or a Modified READ coding method (called a MH coding method and a MR coding method, respectively, hereinafter), both of which can compress and encode the data for transmission by utilizing the high correlation between pels, and which are recommended by the International Telegraph and Telephone Consultative Committee (CCITT) as a standard coding method of two-level, i.e. black and white picture signals.
To overcome this problem, there is proposed a method as disclosed in the paper "MODIFIED HUFFMAN CODING FOR HALF-TONE PICTURES" by Takao Omachi et al, the conference record of the IEEE International Conference on Communications, 1985, pp 1410 to 1414. According to this paper, there is provided a preprocessor before a MH or MR coding apparatus in order to efficiently compress the dither signals, in which a dither signal z.sub.j of a pel j to be encoded at present is predicted by using the dither signals of pels which have been already encoded and transmitted prior to the prediction of the pel j.
In the prior art described above, the correlation between picture signals of pels must be obtained by the statistical measures in advance and stored as a table. Therefore, there occurs a new problem that the data compression efficiency varies to a great extent in accordance with the nature of a picture to be processed. Further, since, as seen from FIGS. 1a to 1c, the correlation between signals of pels is different every pattern of arranging the thresholds in the dither matrices, the statistical measures must be conducted by the number of times corresponding to the number of the dither matrices prepared, and also the tables for storing the results of the statistical measures must be provided as much.